Detailed Instructions: Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.
Problem:How should you reply to 'what's up?'.
Solution:
'What's up' means 'What's happening.' I usually just reply 'nothing.' because nothing is happening to me. But, there are alternatives, such as the usual reply to a greeting: Not much Nothing. Yourself? I'm good, yourself? Good. How are you? However, if there is something that you would like to tell the other person e.g. You've sold your car, it might go like this: A: What's up? B: I've sold my car. A: Good on you mate!